class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) 
    {
        ListNode* head = new ListNode(-1);
        ListNode* ret = head;
        
        while(l1 != NULL && l2 != NULL)
        {
            if((l1 -> val) < (l2 -> val))
            {
                head -> next = l1;
                l1 = l1 -> next;
            }
            
            else
            {
                head -> next = l2;
                l2 = l2 -> next;
            }
            
            head = head -> next;
        }
        
        head -> next = l1 == NULL ? l2 : l1;
        return ret -> next    ;
    }
};